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Decimate's Un-Subdivide mode no longer works
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Description

System Information
Operating system: Darwin-18.7.0-x86_64-i386-64bit 64 Bits
Graphics card: AMD Radeon Pro 580 OpenGL Engine ATI Technologies Inc. 4.1 ATI-2.11.20

Blender Version
Broken: version: 2.81 (sub 16), branch: master, commit date: 2019-12-04 11:32, hash: rBf1aa4d18d49d
Worked: 2.80

Short description of error
Decimate's Un-Subdivide mode no longer works.
I used to use Un-Subdivide a lot in Blender 2.8, but it has stopped working in 2.81.

Exact steps for others to reproduce the error
Create a plane.
Subdivide it a few times.
Add the Decimate modifier.
Switch to Un-Subdivide mode.
Increase iterations.
Notice no effect.

Event Timeline

Cannot confirm on Linux, Decimate's Un-Subdivide behaves the same between 2.79, 2.80 and 2.81 here.

I can't reproduce it here in windows either. It works fine..

Philipp Oeser (lichtwerk) changed the task status from Needs Triage to Needs Information from User.Jan 6 2020, 11:23 AM

Also cannot reproduce.

Just to make sure:

  • Could you share your .blend file with the decimate modifier?

Uploaded blend file.

  1. Created plane.
  2. Pressed tab to go to edit mode on plane.
  3. Right-click, click Subdivide.
  4. Increased "Number of Cuts" to 10.
  5. Pressed Command-T to convert all quads to triangles.
  6. Pressed tab to exit edit mode.
  7. Switched to wireframe mode to see polygons.
  8. Added "Decimate" modifier.
  9. Switched Decimate mode to Un-Subdivide.
  10. Increased "Iteration" and saw no difference.

...

  1. Pressed Command-T to convert all quads to triangles.

...

The modifier won't work with such topology:

It is intended for meshes with a mainly grid-based topology (without giving uneven geometry). If additional editing has been done after the subdivide operation, the results may be unexpected.

This is not a bug.

All I know is that it worked before I upgraded to 2.81.

Neither 2.79b, nor 2.80 un-subdivide such a triangulated plane. As the documentation says, you may get some results from a non-grid, non-quad geometry, but you can't rely on that.

I guess it's possible that I was working with Un-Subdivide-friendly meshes before, but it seems very odd that I would have so much success with it then suddenly no success.